Description: Highlights • Individual evolution of temporal and spatial co-existing magma suites • Determination of pre-eruptive magma chamber conditions of the Cão Grande Formation magma chambers • Cão Grande Formation phonolite magmas typically reach H2O-saturation prior to the eruption. Abstract The Cão Grande Formation (CGF) on the western plateau of Santo Antão is a sequence of four phonolitic tephras (Canudo Tephra, Cão Grande I Tephra, Cão Grande II Tephra and Furninha Tephra) produced by highly explosive eruptions that alternatingly originated from a basanitic - phonolitic and a nephelinitic - phonolitic magmatic system. Detailed stratigraphy and petrological investigations of each unit are used to demonstrate the unusual situation that two distinct highly evolved magmas differentiated contemporaneously in separate magmatic systems. Chemical thermobarometry suggests that both magmatic systems not only temporally co-existed, but also that their magma chambers resided close to each other at 7 to 16 km depth, beneath the western plateau of Santo Antão. However, the distinct melt and magma compositions indicate that both systems evolved independently. The only interaction between both magmatic systems was an injection of magma from the nephelinitic - phonolitic magmatic system into the Cão Grande II Tephra (CG II) phonolitic reservoir, which is associated to the basanitic - phonolitic magmatic system. Compositional zonations in the tephra deposits indicate that the eruptions of the CGF tapped stratified magma reservoirs that mainly resulted from crystal accumulation generating downward increasing magma density. However, the CG II tephras also show a significant gradient in melt (glass) compositions. Magmas of the Canudo Tephra (CT) and the Cão Grande I Tephra (CG I) were H2O-saturated and their eruptions were probably triggered by fluid overpressure in the magma chamber. On the other hand, the CG II magma was H2O-undersaturated; we therefore assume that the injection of the hot nephelinitic - phonolitic magma system-type melt/magma triggered the eruption. The zoned deposit of the Furninha Tephra (FT) indicates mafic magma replenishment into a phonolitic reservoir directly prior to the eruption, thus providing a probable triggering mechanism. The new magma chamber models and thermobarometric results for the four CGF units provide constraints for hazard assessments, because similar events may occur in the future considering the longevity of the CGF magma systems.

Description: During IODP NanTroSEIZE Expedition 322, four packages of tuffaceous sandstones (TST 1, 2, 3a, 3b) were recovered within a moderately lithified and bioturbated silty claystone succession in the Late Miocene (〉 7.6 to ~ 9.1 Ma) upper part of the middle Shikoku Basin deposits. To assess the emplacement processes of the tuffaceous sandstones we investigate modal and geochemical compositions of 24 thin sections that reveal systematic vertical changes within each bed. TST 1, 2 and 3b are single beds whereas TST 3a is composed of at least two beds suggesting several rapidly succeeding sedimentation events. The beds are density-graded such that low-density pyroclasts including pumice lapilli are enriched at the top whereas dense lithic components and minerals are enriched at the bottom. The volcanic glass particles (pumice and shards) that are the dominant modal constituents of each sandstone bed have homogeneous geochemical compositions in each bed. Moreover, TST 1, 2, and 3a glass compositions overlap completely but TST 3b glass has a different composition, as is analogously observed for the mineral compositions. This unique multistage approach of sedimentological and geochemical methods facilitated the detailed investigation of distal volcano-derived, probably tsunamogenic, turbidites in order to contribute to the distinction between primary and secondary induced mass flows. We interpret that all tuffaceous sandstones were emplaced by turbidity currents that were formed during major explosive volcanic eruptions. However, while TST 1, 2, and 3a turbidity currents formed by the entry into the ocean of voluminous pyroclastic flows erupted at a volcano on mainland Japan, TST 3b was emplaced by a turbidity current formed by a shallow submarine or subaerial volcanic eruption at the Izu–Bonin arc where it collided with Japan. These results regarding distal turbidites encourage the revisiting of older marine deposits in the scope of hazard evaluation through past events, especially in regions near to volcanic sources.

Description: Highlights • Subplinian to Plinian eruptions from Cocos Island • Tectonically controlled melt ascent • Ocean island evolution without passing typical growth stages Abstract We report a series of fourteen marine tephra layers that are the products of large explosive eruptions of Subplinian to Plinian intensities and magnitudes (VEI 〉 4) from Cocos Island, Costa Rica. Cocos Island is a volcanic island in the eastern Central Pacific Ocean ~ 500 km offshore Costa Rica, and is situated on the northwestern flank of the aseismic Cocos Ridge. Geochemical fingerprinting of Pleistocene (~ 2.4–1.4 Ma) marine tephra layers from Ocean Drilling Project (ODP) Leg 202 Site 1241 using major and trace element compositions of volcanic glass shards demonstrates unequivocally their origin from Cocos Island rather than the Galápagos Archipelago or the Central American Volcanic Arc (CAVA). Cocos Island and the adjacent seamounts of the Cocos Island Province have alkalic compositions and formed on young (≤ 3 Ma) oceanic crust from an extinct spreading ridge bounded by a transform fault against the older and thicker crust of the aseismic Cocos Ridge. Cocos Island has six times the average volume of the adjacent seamounts although all appear to have formed during the 3–1.4 Ma time period. Cocos Island lies closest to the transform fault and we explain its excessive growth by melts rising from garnet-bearing mantle being deflected from the thick Cocos Ridge lithosphere toward the thinner lithosphere on the other side of the transform, thus enlarging the melt catchment area for Cocos Island compared to the seamounts farther away from the transform. This special setting favored growth above sea level and subaerial explosive eruptions even though the absence of appropriate compositions suggests that the entirely alkalic Cocos Island (and seamounts) never evolved through the productive tholeiitic shield stage typical of other Pacific Ocean islands, possibly because melt production rates remained too small. Conditions of magma generation and ascent resembled Hawaiian pre-shield volcanoes but persisted for much longer (〈 1 m.y.) and formed evolved, trachytic magmas. Therefore Cocos Island may be a unique example for a volcanic ocean island that did not pass through the typical growth stages.

Description: Small-volume (ca. 0.6 km3) pyroclastic flow deposits at Laacher See contain lithic breccias and two types of ground layers that differ significantly in their structure and composition from the main body of flow units. Lithic breccia bodies, up to 3.5 m thick, containing up to 85 weight% lithic blocks, occur locally at various distances from the vent. The deposition of these breccias was apparently governed by the strong influence of paleomorphology on the dynamics of the pyroclastic flows. The breccias were deposited at three main changes in bottom gradient along the path of the pyroclastic flows. The accumulation of large lithics is explained: (a) by compression of flows on the rising bottom close to the vent; (b) by thinning of flows accelerating over a steep incline; (c) by deceleration of the pre-concentrated lower part of flows in hydraulic jumps; and (d) possibly by a stationary vortex at the inner bend of a valley curvature. Poorly sorted lithic-rich ground layers, laterally highly variable in internal structure and composition, are restricted to marginal regions of the pyroclastic flow deposits within deep and narrow valleys. They are interpreted as having formed due to the extreme roughness of the valley walls, enforcing irregular turbulent flow and intense fluidization of the flow head, in which density-dominated segregation of lithics occurred. Wellsorted lapilli-rich ground layers of constant lateral thickness were probably generated by a more regularly moving, less intensely fluidized head of pyroclastic flows in which size-dominated segregation was effective but density-segregation was minor. A model of the temporal and longitudinal evolution of a flow head is proposed. Close to the vent, the head is exclusively erosive. With increasing distance, erosive power declines and erosion is paralleled by ground layer formation under strong fluidization. Further from the vent, the head ceases to erode while fluidization is still sufficient for ground layer formation. When fluidization declines to a level ineffective for segregation, ground layers terminate while the head advances and only terminates when plug-flow dominates.

Description: The dacite to andesite zoned Mateare Tephra is the fallout of a predominantly plinian eruption from Chiltepe peninsula at the western shore of Lake Managua that occurred 3000–6000 years ago. It comprises four units: Unit A of high-silica dacite is stratified, ash-rich lapilli fallout generated by unsteady subplinian eruption pulses affected by minor water access to the conduit and conduit blocking by degassed magma. Unit B of less silicic dacite is well sorted, massive pumice lapilli fallout from the main, steady plinian phase of the eruption. Unit C is andesitic fallout that is continuous from unit B except for the rapid change in chemical composition, which had little influence on the ongoing eruption except for a minor transient reduction of the discharge rate and access of water to the conduit. After this, discharge rate re-established to a strong plinian eruption that emplaced the main part of unit C. This was again followed by water access to the conduit which increased through upper unit C. The lithic-rich lapilli to wet ash fallout of unit D is the product of the fully phreatomagmatic terminal phase of the eruption. A massive well-sorted sand layer, the Mateare Sand, replaces laterally variable parts of unit A and lowermost part of unit B in outcrops up to 32 m above present lake level. The corresponding interval missing in the primary fallout can be identified by comparing the composition of pumice entrained in the sand, and pumice from the local base of unit B on top of the sand, with the compositional gradient in undisturbed fallout. The amount of fallout entrained in the sand decreases with distance to the lake. The Mateare Sand occurs at elevations well above beach levels and its widespread continuous distribution defies a fluviatile origin. Instead, it was produced by lake tsunamis triggered by eruption pulses during the initial unsteady phase of activity. Such tsunamis could threaten areas not affected by fallout, and represent a hazard of particular importance in Nicaragua where two large lakes host several explosive volcanoes.

Description: Highlights: • Two new phonolitic tephra units complementing the two previously known. • First radiometric ages of the CGF. • Contemporaneously evolution of the CGF and the Tope de Coroa. • Marine correlations improve tephra volume estimations for CG I and II. Abstract: The Cão Grande Formation (CGF) on the western plateau of Santo Antão Island is part of the younger volcanic sequence that originated from both, basanitic and nephelinitic magmatic suites, respectively called COVA and COROA suites. Based on our detailed revised stratigraphy of the CGF, including two yet unknown tephra units, we can show that both suites produced multiple, highly differentiated eruptions over a contemporaneous period. Correlations of CGF tephras with marine ash layers provide distal dispersal data for Cão Grande I (CG I) and also identify two highly explosive, phonolitic eruptions that pre-date the CGF tephra deposits known on land. Within the CGF, the lowermost, 220±7 ka old unit Canudo Tephra (CT; COVA suite) comprises phonolitic fall deposits and ignimbrites; it is partly eroded and overlain by debris flow deposits marking a hiatus in highly differentiated eruptions. The phonolitic CG I Tephra (COROA suite) consists of an initial major plinian fall deposit and associated ignimbrite and terminal surge deposits. This is immediately overlain by the phonolitic to phono-tephritic Cão Grande II (CG II; COVA suite), a complex succession of numerous fallout layers and density-current deposits. CG I and CG II have radiometric ages of 106±3 ka and 107±15 ka, respectively, that are identical within their error limits. The youngest CGF unit, the Furninha Tephra (FT; COROA suite), consists of three foidic-phonolitic fall deposits interbedded with proximal scoria deposits from a different vent. The phonolitic eruptions switched to and fro between both magmatic suites, in each case with a stronger first followed by a weaker second eruption. Each eruption evolved from stable to unstable eruption columns. During their terminal phases, both magma systems also leaked evolved dome-forming lavas next to the tephras. Distal ashes increase the CG I tephra volume to ~ 10 km3, about twice the previously published estimate. The tephra volume of CG II is ~ 3 km3; CT and FT are too poorly exposed for volume estimation. The characteristics of the CGF tephra units outline hazard conditions that may be expected from future evolved explosive eruptions on the western plateau of Santo Antão.

Description: Highlights • Data set of Br and Cl emissions from 29 large CAVA eruptions (VEI 〉 5). • Melt inclusions are strongly enriched in chlorine and bromine compared to their respective matrix glasses. • Fluid partitioning is 4 to 68 times more efficient for Br than for Cl. • Subducted calcareous sediments are the major control on the arc-magmatic bromine contents. • Average CAVA eruption would add 368% EESC of recent annual loading to stratosphere. Abstract Large explosive volcanic eruptions inject gases, aerosols, and fine ashes into the stratosphere, potentially influencing climate and atmosphere composition on a global scale. Although the potential climate effect of chlorine (Cl) and bromine (Br) injections into the stratosphere is known, the global mass fluxes are poorly constrained. In this study we focus on the magmatic degassing systematics and budgets of Br and Cl, and on constraining the major sources of Br in a subduction setting. We therefore present a regional time series of Br and Cl emissions from 29 highly explosive eruptions throughout the Central American Volcanic Arc (CAVA), covering the last 200 ka, and a range of magmatic compositions and eruption magnitudes. We have measured Br and Cl in matrix glasses and melt inclusions using synchrotron radiation micro X-ray fluorescence spectrometry (SR micro-XRF) and electron microprobe, respectively. Melt inclusions of the CAVA tephras generally have higher Br (0.9 to 17.9 ppm) and Cl (770 to 3800 ppm) contents than the matrix glasses (0.39 to 1.5 ppm Br, 600 to 2800 ppm Cl). Moreover, the difference between maximum and minimum concentrations observed in melt inclusions of a given sample ranges between 9 and 90% of the maximum observed concentration for Br, and between 2 and 40% for Cl. Such intra-sample variations arise from variable pre-eruptive degassing of these halogens into a magmatic fluid phase. The relative loss of Br from the melt is 4 to 68 times higher than that of Cl. The masses of Br (2–1100 kt) and Cl (0.1 to 800 Mt) emitted by the eruptions generate instantaneous additions to the stratosphere potentially amounting to ∼6–5600% of the present-day stratospheric annual global loading of Equivalent Effective Stratospheric Chlorine. As the size of the stratospheric impact is primarily a function of eruption magnitude, we use magnitude-frequency relationships to estimate that eruptions adding ∼10% to resident EESC loading would occur every 〈40 years while every ∼200 years an eruption would double the EESC loading. Comparing the variations in Br and Cl concentrations and particularly minimum Cl/Br ratios in melt inclusions with geochemical trace-element proxies (e.g. U/La, Ba/Th) and lead-isotope compositions, which change along the arc in response to changing subduction conditions, we suggest that subducted calcareous sediment is a major source of magmatic Br but also infer an important role of fluids expelled from serpentinized subducted mantle. Extrapolation of CAVA volcanic Br emissions to the global subduction system thus needs to consider variations in the nature of subducted lithologies.

Description: Thirteen sediment gravity cores from the seafloor around the southern Cape Verdean islands Fogo and Brava as well as the Cadamosto seamount recovered 43 mafic and 5 phonolitic Pleistocene to Holocene primary ash layers. Twelve of these layers could be identified in several cores; they evidently cover areas of at least 6200-17,650km2 corresponding to minimum tephra volumes of ~1km3 (Volcanic Explosivity Index (VEI) 5), and thus are attributed to eruptions of sub-Plinian to Plinian dimensions. Provenance analyses based on geochemical compositions and geological evidence link the mafic tephras to eruptions on Fogo island. The detailed foraminiferal δ18O stratigraphy of one core yields a high-resolution age scale that can be applied to other cores via correlated ash beds and background sediment intervals. Sedimentation rates of the background sediment then constrain ages of other ash beds in the cores. The resulting temporal distribution of the marine tephras indicates that relatively large magnitude, highly explosive eruptions occurred about every 3000years on Fogo during the last 150kyrs; during the past 30kyrs the rate has increased to one in 2000years. In addition, we identified a 117kyr old thick turbidite sequence containing both mafic and phonolitic ash. We interpret this as the deposit of density currents generated by the Monte Amarelo flank collapse on Fogo and its associated tsunami.On the other hand, only a single widespread phonolitic tephra layer (145. kyrs old) could be correlated to Brava island indicating that highly explosive activity, producing deposits spread widely beyond the island's shores, did not occur anymore over the last 145. kyrs. No equivalents of the caldera filling ignimbrites on Brava could be found in the cores and therefore we infer that the caldera formed earlier than the 155. ka reached by coring. Two widespread phonolitic tephra layers, 17. ka and 40. ka old, are correlated to the Cadamosto seamount and thus derived from eruptions that occurred at 〉. 1380. m water depths where thermal granulation producing blocky glass shards was a major fragmentation process. We interpret that these eruptions produced huge submarine volcanic ash-water plumes that spread widely across the seafloor.In conclusion, our marine tephrostratigraphy provides the first evidence of frequent highly explosive volcanic eruptions at the southwestern part of the Cape Verdes during the past 155. kyrs. A somewhat surprising result is that such large eruptions occurred much more frequently from mafic alkalic magmas at Fogo than from highly evolved phonolitic magmas on Brava.

Description: Highlights • Temporally close-spaced double eruption within a couple of hundreds of years. • Magmas are variably tapped from zoned magma chambers during eruptions due to changing magma discharge rates and/or vent migration. • Eruptions started with a series of fallouts featuring stable eruption columns followed by fluctuating and partially collapsing eruption columns. • Eruptive volumes sum up to a total of 25.6 km3 and 40.5 km3 tephra volume, eruption column heights have been between 20–33 km. • Potential hazards from similar sized eruptions around Coatepeque Caldera are indicated even in the distal regions around San Salvador. Abstract The Coatepeque volcanic complex in El Salvador produced at least four Plinian eruptions within the last 80 kyr. The eruption of the 72 ka old Arce Tephra formed the Coatepeque Caldera and was one of the most powerful explosive eruptions in El Salvador. Hitherto it was thought that the Arce tephra had been emplaced only by one, mostly Plinian, eruptive event that ended with the deposition of a thick ignimbrite. However, our stratigraphic, geochemical, and zircon data reveal a temporally closely- spaced double eruption separated by a gap of only a couple of hundred years, and we therefore distinguish Lower and Upper Arce Tephras. Both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions. The final phase of the Upper Arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse. Mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the Pacific Ocean, the Guatemalan lowlands and the Caribbean Sea, result in 25.6 km3 tephra volume, areal distribution of 4 × 105 km2 and eruption column heights between 20–33 km for the Lower Arce eruption, and 40.5 km3 tephra volume, including 10 km3 for the ignimbrite, distributed across 6 × 105 km2 and eruption column heights of 23–28 km for the Upper Arce eruption. These values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at Coatepeque Caldera, especially for this kind of double eruption.